This disclosure is directed to an improved low refractive index coating for optical fibers, methods of manufacture thereof and to articles comprising the same. In particular, this disclosure relates to fluorinated low refractive index coatings for optical fibers that do not contain long-chain molecules with tri-fluorinated carbon atoms (CF3) and that are devoid of the following types of fluorinated molecules:                CF3(CF2)n—CH═CH2;        CF3(CF2)n—C(═O)—X where X is any chemical moiety;        CF3(CF2)m—CH2—X where X is any chemical moiety;where n>5 or m>6.        
Fluorinated low refractive index coatings for optical fibers have previously been achieved using perfluorinated polyether (PFPE) oligomers. Principle examples are UV-curable derivatives of ethoxylated PFPE diols that are sold by Solvay under the tradenames FLUOROLINK® and FOMBLIN® and that have the following general structure:X—CF2—O—(CF2—CF2—O)p—(CF2O)q—CF2—Xwhere the X end groups typically contain one or more —CH2 bridging groups and as well as one or more alcohol (—OH) moieties that can be derivatized (e.g., to become acrylated). These PFPEs generally have relatively high number average molecular weights (on the order of 1500-2600 grams per mole) that are often leveraged to attain “high” viscosity and enable desirable convenient usage of conventional high-pressure optical fiber coating applicators. Desirably, PFPEs inherently have low refractive indices, but their use has certain disadvantages, chief amongst these being that PFPEs are themselves soft and do not impart hardness where hardness may be desirable. PFPE derivatives, e.g., PFPE (meth)acrylates or urethane (meth)acrylates, display low thermo-oxidative stability (after curing) in comparison with fluorocarbon materials that are not polyether based. In addition, PFPE oligomers can be more difficult to purify than lower molecular weight fluorinated monomers and some evidence suggests that hydrophilic impurities contribute to poor performance of PFPE-based claddings in humid environments. The term (meth)acrylate as used herein refers to either a methacrylate or to an acrylate.
PFPE derivatives, by virtue of their high molecular weight, are characterized by significant polydispersity; their molecular weight distribution can be difficult to control and reproduce when manufacturing successive lots. The polydispersity can in turn cause lot-to-lot variability in viscosity and in the compatibility with other coating components; such variability can reduce the consistency of fibers made using PFPE-based polymer claddings.
Other commercial polymer claddings have relied upon chemical components whose usage is being eliminated or severely restricted by chemical regulatory agencies (i.e., structures having similarity to perfluorinated octanoic acid—PFOA—as well as its precursors and higher homologues). PFOA has been found to be biopersistent and bioaccumulative. While not all components of the current invention have been tested for biopersistence and bioaccumulative propensity, it is a goal to not utilize components that are described by the US EPA's Long-Chain Perfluorinated Chemicals (PFAC) Action Plan (2009).
It is therefore desirable to use environmentally friendly coatings that have low refractive indices, a lower susceptibility to oxidative degradation, and a more consistent production profile (i.e., minimal property differences due to changes in manufacturing conditions). In addition, it is desirable to use coatings that have a suitable balance of properties including hardness (or modulus) and refractive index.